The present invention relates to light imaging detection and ranging (LIDAR) systems, and more particularly to an integrated LIDAR receiver that provides both imaging and ranging information, and wherein the ranging information is obtained using a pickoff circuit that does not interfere with the optimum biasing of the LIDAR receiver.
In a LIDAR system, a pulsed light source (such as a pulsed laser) emits a brief intense illuminating pulse of light at a known time, e.g., when triggered by a timing circuit. The pulse of light is directed at a target area where a specific target, such as a rocket, is to be monitored. The emitted light pulse propagates at the speed of light (for the relevant medium through which it is traveling), and thus arrives at the target some finite time after its known transmission time. Upon striking the target, a portion of the energy associated with the incident light pulse is reflected from the target. The reflected light pulse also propagates at the applicable speed of light back to a receiver location, where an appropriate receiver is positioned to receive the reflected light energy. Thus, the light travels the same path going to the target as it does returning therefrom, and the propagation time is the same in both directions. Advantageously, the returned light pulse includes both ranging information (i.e., the round trip propagation time of the emitted light pulse) and imaging information (i.e., whatever image information is recoverable from the returned light pulse) for the target. Thus, through the use of appropriate processing circuits within the receiver, coupled to the same timing circuit used to trigger the light pulse, it is possible to extract both the ranging and imaging information from the returned light pulse.
If a perfect light detector were available, the extraction of the desired ranging and imaging information from the returned light pulse would be straightforward. That is, all that would be required would be to position such "perfect light detector" to detect the received light pulse, note the precise time at which the returned light pulse was received relative to when the light pulse was transmitted in order to determine the ranging information, and to process the detected light signal in order to provide the desired imaging information.
Unfortunately, perfect light detectors do not exist. All known light detectors use an appropriate conversion process to convert the incident photon energy to an appropriate output signal, usually an electrical signal, and/or to an amplified photon signal. That is, light detectors utilize a conversion process that converts incident photons (contained in the incident light signal) to electrons (contained in an electrical output signal). Further, an amplified (or intensified) photon signal is generally obtained only after first converting the photons to electrons, amplifying the electrons, and converting the amplified electrons back to photons.
Disadvantageously, conflicting requirements exist relative to the type of photon conversion process needed to provide the best ranging information and the best imaging information. For ranging, it is best to have a very sharp return pulse, indicative of the precise time a packet of photons is received at the light detector. Such sharp return pulse thus advantageously provides a precise indication of when the return pulse was received, and thus provides (when compared to the trigger time of when the initial pulse was transmitted) a very accurate indication of the round trip signal propagation time, and hence an accurate indication of the location of the target relative to the LIDAR transmitter/receiver location.
In order to detect such a sharp return pulse, useful for ranging purposes, a wide bandwidth is required. Unfortunately, imaging detectors do not have a sufficiently wide bandwidth for this function. That is, by the time the photons have been received and processed so as to provide an image signal in imaging detectors of the prior art, too much time has elapsed for the signal to be an accurate indicator of the photon receipt time.
In order to address the above conflicting photon detection requirements, it is known in the art to use a LIDAR receiver that has two different detectors, one for ranging and one for imaging. For such dual-detector LIDAR receiver to function, it is necessary to split the return light pulse (or return "beam"). One part of the split beam is directed to a first detector, typically an avalanche photodiode, used to measure the time-of-flight for range determination. The other part of the split beam is directed to a gated image intensifier (amplifier), which intensifies (amplifies) the returned image. The intensified image is then directed to an imaging device, typically a charge-coupled device (CCD) camera, whereat it is digitized for image processing.
The dual-detector LIDAR receiver requires two optical paths. Disadvantageously, the return beam power must be split or divided between the two paths, thereby reducing the available power in each path. Reduced power, in turn, lowers the overall range over which the LIDAR receiver may be used. What is needed, therefore, is a LIDAR receiver having increased range capabilities.
The use of two optical paths also requires the use of several optical components, e.g., a beamsplitter, optical filters and focusing optics, in each optical path. Disadvantageously, for airborne applications, such as military or civilian satellite use, or for use within other types of spacecraft, such optical components significantly add to the size, weight and cost of the LIDAR receiver. What is needed, therefore, is a LIDAR receiver that provides the desired imaging and ranging information without the need for separate optical paths, thereby allowing the size, weight and cost of the LIDAR receiver to be significantly reduced.
In dual-detector LIDAR receivers of the prior art, an image intensifier is used in one of the optical paths to bring the intensity of an image to a level that allows it to be sensed by an imaging detector, such as a CCD camera. See, e.g., U.S. Pat. No. 3,128,408 for a description of a basic image intensifier. The image intensifier may be a proximity focused image intensifier, which includes an opaque photocathode, a microchannel plate (MCP) electron multiplier, and a phosphor coated anode. The photocathode is electrically biased so as to accelerate electrons toward the MCP. The MCP has an electrical bias applied thereto that allows electrons flowing thereacross to be multiplied by a factor of 100 to 100,000. The phosphor coated anode and exit side of the MCP are biased so as to accelerate electrons toward the phosphor anode. When the electrons strike the phosphor, the phosphor is excited and photons are emitted, thereby providing an image--an intensified image--of the incoming optical signal. Unfortunately, the time delay between the instant the electrons strike the phosphor and the time that an image is formed and may be processed by a suitable CCD camera is too long to allow the image to be used for ranging purposes. In other words, the CCD camera has too low of a bandwidth to provide useful ranging information.
In the other optical path of the dual-detector LIDAR, a silicon avalanche photodiode (APD) is used as the ranging detector. Advantageously, the APD provides one of the highest sensitivities of known photodetectors. However, in order to achieve its highest gain, an APD must be biased at levels that increase its "popcorn noise" and other noise, thereby limiting the effective sensitivity that the APD could otherwise achieve. Hence, what is needed is a photodetector that may be used to provide ranging detection within a LIDAR receiver that offers the same or better sensitivity as an APD detector, but without the limitations of the APD when used at the bias voltages required within a LIDAR receiver.
Thus, in summary, there is an unfilled need in the art for an imaging and ranging LIDAR receiver that offers higher sensitivity, lighter weight, more compact size, and lower cost than has heretofore been achievable. The present invention advantageously addresses these and other needs.